The proposed research is designed to describe in detail the relationships between different classes of axon terminals and specific neurons in the cat medial superior olivary nucleus using a variety of methods including electron microscopy of Golgi impregnated material. New methods have been developed for incubating fresh brain stem slices, fixing and embedding them in plastic and performing light and electron microscopic autoradiography on sections. These methods will allow a variety of auditory areas to be surveyed light microscopically to determine the differential ability of cells and cellular elements to sequester labeled compounds including H3-amino acids putative neurotransmitters (i.e., glycine, glutamic acid, aspartic acid and GABA). Areas which sequester sufficiently large concentrations of labeled compounds can then be studied electron microscopically to correlate the ultrastructural and transmitter related properties of synaptic terminals. Genetically caused alterations of the auditory system, especially brain stem nuclear groups, are being described in deaf white cats and C57BL6 mice.